This section introduces aspects that may be helpful to facilitating a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
To drop just a single wavelength of a multi-channel optical signal at a receiver, current metro architectures sometimes use a fixed or reconfigurable drop optical filter. Dropping a channel is typically required for non-coherent receivers, which cannot accept more than one wavelength. The use of fixed filters in the drop direction results in a static coloring arrangement, which does not provide sufficient flexibility for some applications. This can be resolved by the use of reconfigurable optical add/drop multiplexer (ROADM) node architectures based on wavelength blockers or wavelength selective switches (WSSs). However, these components can add considerable infrastructure cost and loss.
Alternatively, the receiver of a coherent optical transponder (OT) can accept multiple wavelength channels and tune to a specific desired channel. However, most optical solutions are still based on either ROADM or simple passive splitter technology. Architectures based on passive splitters are viable for small networks, but are usually too limited in the number of services they can support, depending on the specifications of the coherent OTs. In the drop direction, there may be a limit to the number of wavelengths that can be dropped to each receiver, due to front-end common-mode rejection ratio (CMRR) and dynamic range restrictions, especially in a low cost network without spectral equalization capabilities for ripple reduction. In the add direction, many of the newer generation of coherent optical transponders contain transmit amplifiers that result in broadband noise that can get added to all other add wavelengths and through-path wavelengths, whether or not they originate at the node in question. This broadband amplified spontaneous emission (ASE) noise can be prefiltered, either on a per-transponder basis or for a small set of transponders, while keeping coloring flexibility, by either the use of tunable optical filters or an add path wavelength-selective switch (WSS), but this may be incompatible with the goal of a low-cost architecture.